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SubjectRe: [RFC PATCH v2 5/6] sched/fair: Select an energy-efficient CPU on task wake-up
On Fri, Apr 06, 2018 at 04:36:06PM +0100, Dietmar Eggemann wrote:
> From: Quentin Perret <quentin.perret@arm.com>
>
> In case an energy model is available, waking tasks are re-routed into a
> new energy-aware placement algorithm. The eligible CPUs to be used in the
> energy-aware wakeup path are restricted to the highest non-overutilized
> sched_domain containing prev_cpu and this_cpu. If no such domain is found,
> the tasks go through the usual wake-up path, hence energy-aware placement
> happens only in lightly utilized scenarios.
>
> The selection of the most energy-efficient CPU for a task is achieved by
> estimating the impact on system-level active energy resulting from the
> placement of the task on the CPU with the highest spare capacity in each
> frequency domain. The best CPU energy-wise is then selected if it saves
> a large enough amount of energy with respect to prev_cpu.
>
> Although it has already shown significant benefits on some existing
> targets, this approach cannot scale to platforms with numerous CPUs.
> This patch is an attempt to do something useful as writing a fast
> heuristic that performs reasonably well on a broad spectrum of
> architectures isn't an easy task. As a consequence, the scope of
> usability of the energy-aware wake-up path is restricted to systems
> with the SD_ASYM_CPUCAPACITY flag set. These systems not only show the
> most promising opportunities for saving energy but also typically
> feature a limited number of logical CPUs.
>
> Moreover, the energy-aware wake-up path is accessible only if
> sched_energy_enabled() is true. For systems which don't meet all
> dependencies for EAS (CONFIG_PM_OPP for ex.) at compile time,
> sched_enegy_enabled() defaults to a constant "false" value, hence letting
> the compiler remove the unused EAS code entirely.
>
> Cc: Ingo Molnar <mingo@redhat.com>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Quentin Perret <quentin.perret@arm.com>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> ---
> kernel/sched/fair.c | 97 ++++++++++++++++++++++++++++++++++++++++++++++++++---
> 1 file changed, 93 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8cb9fb04fff2..5ebb2d0306c7 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6700,6 +6700,81 @@ static unsigned long compute_energy(struct task_struct *p, int dst_cpu)
> return energy;
> }
>
> +static int find_energy_efficient_cpu(struct sched_domain *sd,
> + struct task_struct *p, int prev_cpu)
> +{
> + unsigned long cur_energy, prev_energy, best_energy, cpu_cap;
> + unsigned long task_util = task_util_est(p);
> + int cpu, best_energy_cpu = prev_cpu;
> + struct freq_domain *fd;
> +
> + if (!task_util)
> + return prev_cpu;
> +
> + if (cpumask_test_cpu(prev_cpu, &p->cpus_allowed))
> + prev_energy = best_energy = compute_energy(p, prev_cpu);
> + else
> + prev_energy = best_energy = ULONG_MAX;
> +
> + for_each_freq_domain(fd) {
> + unsigned long spare_cap, max_spare_cap = 0;
> + int max_spare_cap_cpu = -1;
> + unsigned long util;
> +
> + /* Find the CPU with the max spare cap in the freq. dom. */
> + for_each_cpu_and(cpu, freq_domain_span(fd), sched_domain_span(sd)) {
> + if (!cpumask_test_cpu(cpu, &p->cpus_allowed))
> + continue;
> +
> + if (cpu == prev_cpu)
> + continue;
> +
> + util = cpu_util_wake(cpu, p);
> + cpu_cap = capacity_of(cpu);
> + if (!util_fits_capacity(util + task_util, cpu_cap))
> + continue;
> +
> + spare_cap = cpu_cap - util;
> + if (spare_cap > max_spare_cap) {
> + max_spare_cap = spare_cap;
> + max_spare_cap_cpu = cpu;
> + }
> + }

If have two clusters, and if firstly iterate the big cluster, then
max_spare_cap is a big value for big cluster and later LITTLE cluster
has no chance to have higher value for spare_cap. For this case, the
LITTLE CPU will be skipped for energy computation?

> +
> + /* Evaluate the energy impact of using this CPU. */
> + if (max_spare_cap_cpu >= 0) {
> + cur_energy = compute_energy(p, max_spare_cap_cpu);
> + if (cur_energy < best_energy) {
> + best_energy = cur_energy;
> + best_energy_cpu = max_spare_cap_cpu;
> + }
> + }
> + }
> +
> + /*
> + * We pick the best CPU only if it saves at least 1.5% of the
> + * energy used by prev_cpu.
> + */
> + if ((prev_energy - best_energy) > (prev_energy >> 6))
> + return best_energy_cpu;
> +
> + return prev_cpu;
> +}
> +
> +static inline bool wake_energy(struct task_struct *p, int prev_cpu)
> +{
> + struct sched_domain *sd;
> +
> + if (!sched_energy_enabled())
> + return false;
> +
> + sd = rcu_dereference_sched(cpu_rq(prev_cpu)->sd);
> + if (!sd || sd_overutilized(sd))
> + return false;
> +
> + return true;
> +}
> +
> /*
> * select_task_rq_fair: Select target runqueue for the waking task in domains
> * that have the 'sd_flag' flag set. In practice, this is SD_BALANCE_WAKE,
> @@ -6716,18 +6791,22 @@ static int
> select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_flags)
> {
> struct sched_domain *tmp, *affine_sd = NULL, *sd = NULL;
> + struct sched_domain *energy_sd = NULL;
> int cpu = smp_processor_id();
> int new_cpu = prev_cpu;
> - int want_affine = 0;
> + int want_affine = 0, want_energy = 0;
> int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING);
>
> + rcu_read_lock();
> +
> if (sd_flag & SD_BALANCE_WAKE) {
> record_wakee(p);
> + want_energy = wake_energy(p, prev_cpu);
> want_affine = !wake_wide(p) && !wake_cap(p, cpu, prev_cpu)
> - && cpumask_test_cpu(cpu, &p->cpus_allowed);
> + && cpumask_test_cpu(cpu, &p->cpus_allowed)
> + && !want_energy;
> }
>
> - rcu_read_lock();
> for_each_domain(cpu, tmp) {
> if (!(tmp->flags & SD_LOAD_BALANCE))
> break;
> @@ -6742,6 +6821,14 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> break;
> }
>
> + /*
> + * Energy-aware task placement is performed on the highest
> + * non-overutilized domain spanning over cpu and prev_cpu.
> + */
> + if (want_energy && !sd_overutilized(tmp) &&
> + cpumask_test_cpu(prev_cpu, sched_domain_span(tmp)))
> + energy_sd = tmp;
> +
> if (tmp->flags & sd_flag)
> sd = tmp;
> else if (!want_affine)
> @@ -6765,7 +6852,9 @@ select_task_rq_fair(struct task_struct *p, int prev_cpu, int sd_flag, int wake_f
> sync_entity_load_avg(&p->se);
> }
>
> - if (!sd) {
> + if (energy_sd) {
> + new_cpu = find_energy_efficient_cpu(energy_sd, p, prev_cpu);
> + } else if (!sd) {
> pick_cpu:
> if (sd_flag & SD_BALANCE_WAKE) { /* XXX always ? */
> new_cpu = select_idle_sibling(p, prev_cpu, new_cpu);
> --
> 2.11.0
>

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